This invention relates generally to a non-invasive automated pipetting method, and more particularly, relates to determining the level of fluid present in a test sample. Level sensing is accomplished by moving a pipettor toward a sample while aspirating air and monitoring for a pressure change within the pipettor. Controlled aspiration of the fluid sample is then performed.
Automated pipetting systems that contact a liquid test sample with an electrode are known. For example, a conducting pipette tip or an electrode adjacent to the pipette tip generates an electrical signal when the conducting pipette tip or the electrode touches the surface of an electrically conducting fluid, such as a buffer solution or serum, plasma or urine sample. These methods involve invasive procedures which suffer from the danger of cross-contamination between test samples. A portion of the first sample clings to the pipette tip or electrode and the next sample to be pipetted becomes contaminated with the first sample when the pipette tip or electrode contacts the second sample. This cross-contamination or carry-over can also occur between assay reagents and between assay reagents and samples. The danger of cross-contamination or carry-over can be reduced by extensive washing of the pipette tip or electrode between each pipetting step, but such washing steps suffer from decreased sample throughput on the automated instrument.
Detecting the surface of a fluid is very important for the precise pipetting of the fluid. Locating the fluid surface permits the controlled immersion of the pipette tip in the fluid. By controlling the depth of immersion of the pipette tip in the fluid, a consistent amount of fluid will cling to the outside of the tip resulting in greater consistency in the total volume dispensed. The use of non-invasive fluid sample surface sensing methods and devices in conjunction with disposable polymeric pipette tips results in such greater control and consistency. Thus, non-invasive fluid sample surface sensing achieves two advantages. First, it eliminates the need to wash the pipette tip between sampling, thereby increasing the through put of the instrument. Second, a non-invasive surface probing method eliminates the potential of sample carry-over.
A non-invasive fluid surface-sensing system were disclosed in U.S. Pat. Nos. 3,474,902 and 3,494,191. This non-invasive fluid surface-sensing system utilizes a method that involves blowing air via a stepper-motor controlled syringe to detect a fluid surface level. This level sensing method can be used in automated pipetting of biological samples. However, air is often blown into the test sample causing bubbles and generating aerosols. In an attempt to minimize bubble creation, the pipettor is moved toward the sample very slowly until the sample surface is detected, and then immediately withdrawn to the end of its travel range. The syringe is then fully dispensed to blow all the remaining air out of the syringe. Finally, the pipettor is returned to the fluid surface and aspiration is commenced.
An object of the present invention is to non-invasively level sense a fluid sample without the need of blowing air through the pipette tip. Another object of the present invention is to aspirate a fluid sample by immersing the pipette tip into the sample at a controlled, minimal depth in order to minimize the amount of sample that clings to the outside of the pipette tip. Yet another object of the present invention is to detect nonhomogeneity, such as clots, bubbles and foam, in the fluid sample. Still other objects of the present invention will be apparent to one skilled in the art.
The present invention offers advantages over known methods of level sensing and aspiration of a fluid sample. Carry-over or cross-contamination between samples and reagents is eliminated by employing a non-invasive method in which no contact is made between the level sense means, such as a pressure transducer, and the test sample. The present invention also has advantages over positive pressure (blowing air) level sense methods. The possibility of bubbles and aerosols is eliminated by the present invention. Also, because the need to reverse the direction of the syringe pump between the level sense step and the aspiration step is eliminated, the instrument throughput is increased. In addition, the present invention eliminates the necessity of withdrawing the pipettor from the sample in order to evacuate the syringe before aspiration again improving the instrument throughput through the elimination of method steps without the creation of bubbles and aerosols.